Astronomers are like forensic investigators. We have all this data taken from the scene of some sort of event, and have to piece together what happened. But those folks on CSI have it easy: they get to actually walk around the scene, poke and prod it, examine various stains, and even take physical evidence back to the lab. Astronomers are stuck standing a quintillion kilometers away, and we only get to see things at one angle.

But oh, what an angle. If it pleases the court, I’d like to enter this evidence for your consideration:

That’s my kind of evidence (click to embiggen). It’s an image of the lovely grand-design spiral galaxy M81, one of the nearest major galaxies to our own. At about 12 million light years away it’s bright enough to be seen in binoculars (and in fact some extremely keen-eyed observers have been able to see it with their unaided eyes). That means it’s close enough to study in detail… and what detail!This image is from the monster Subaru telescope, an 8.2 meter (27 foot!) goliath at the peak of the Mauna Kea volcano in Hawaii. The purpose of the image was to examine the galaxy’s halo, the vast region of stars surrounding the main body of the galaxy itself. The current thinking is that big galaxies like M81 and our own Milky Way form when smaller galaxies collide. We have a lot of evidence for this in our galaxy — we’re in the process of eating and digesting several small galaxies even as you read this — but the exact process is unclear.

That’s why we look at galaxy halos. When galaxies collide, a lot of stars get kicked into higher orbits, propelling them out of the main body of the wreck and into the halo. If this were a crime procedural drama, you could think of these stars as blood spatters seen on walls, pointing back silently to the method of the crime, giving the investigators clues on how they got there.

The thing is, the halo of M81 is weird. You might think it would look a lot like the Milky Way’s halo: the masses of the two galaxies are roughly the same, and we expect they formed the same way. But the population of stars surrounding M81 is different! For one thing, these new Subaru images indicate that our neighbor’s halo is much brighter than expected. Also, the stars have more heavy elements than expected — that is, more elements like oxygen, iron, and so on (heavy elements, to an astronomer, are essentially any other than hydrogen and helium).

What does this mean? That part’s not so easy to understand. Maybe the smaller galaxies that wrecked into each other were different than the ones that eventually built up the Milky Way. Maybe the halo was disturbed by the near-collision of M81 with the galaxy M82, a cigar-shaped mess of a galaxy that clearly interacted with M81 a few hundred million years ago (seen here in the ultraviolet in a GALEX image). M81 is also odd in that it has roughly the same mass as the Milky Way, but is not even half its size. Is that another clue to how the mini-galaxies collided to build up M81 billions of years ago?

In fact, the halo of M81 is so odd that astronomers are rethinking what they call a halo. It’s not even clear if it’s just an extension of the main body of the galaxy, or a separate entity unto itself. Obviously this will take a lot more study. And we need a lot more evidence. Blood spatters are critical clues, but to pin the crime on the perp you could also use DNA, fibers, fingerprints… and, of course, a motive would be handy to understand as well.

Sometimes, rounding up the usual suspects simply isn’t enough. Even — especially — when your neighbor is not the kind of person you’d think could do such a thing. M81 always seemed polite, quiet, keeping to itself… but we know how that goes.

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“M81 is also odd in that it has roughly the same mass as the Milky Way, but is not even half its size.”

I presume that means it’s only 50,000 LYs across? Same mass might imply more Dark Matter?

All that metalicity implies many more super nova events (than the Milky Way), which suggests what? Maybe it collided with an anti-matter galaxy(woo hoo!). I know that every electron/positron interaction releases a 2.2 MEV gamma ray, which is a very identifiable signature but we would have to have been around when the wave fronts arrived here to detect that.

Interesting event, from a forensic POV.(Ok, interesting from ANY POV).

What about dark matter? It must be taken into account when two galaxies collide because after all it has the bulk of the galaxies mass. These halos can actually point to where the dark matter is in or around the galaxy in question.

Y’all knew that ‘Subaru’ is Japanese for the Plieades (a.k.a. the “Seven Sisters”, a.k.a. Messier 45 too) right?

For one thing, these new Subaru images indicate that our neighbor’s halo is much brighter than expected.

More stars in the halo than relative to our Milky Way?

Do we have any rough percentages here on what the halo population is in the Milky Way vs M81? (Eg. 5 % MW vs 25% M81 or 35 % MW Vs 55 % M81 etc .. or what? NB. EXAMPLE no.s only – I’ve no idea what the correct figures are!)

Also, the stars have more heavy elements than expected — that is, more elements like oxygen, iron, and so on

Does that make them population zero or minus one stars then? * 😉

M81 is also odd in that it has roughly the same mass as the Milky Way, but is not even half its size.

Size in what respect? Diameter I presume? Obviously not mass since that’s about the same, number of stars? Galactic radius? Pleaes can you specify what you meant here for us BA?

(As #2 Gary Ansorge also pointed out.)

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* A la (as I understand it) – Population I stars in our Galaxy being the metal-rich disk stars, Population II being the older, metal poor cluster (ie,. globular cluster ) & halo stars.
Population III being the oldest least “metallic” first generation of stars ever formed.

Saw you last night on the Discovery Channel”s “How the Universe Works” program. I was happy to see that you were so prominently featured throughout the show. Great job. Can’t wait to see the next episode.

Also note the presence of Messier 82 – the unusual looking edge on spiral above M81 – in that circle field for the lower image.

M82 is a nearby, very dramatic starburst galaxy once thought to have been in the process of exploding and looking a bit like I imagine a quasar might appear if seen from that sort of distance only much dimmer!*

G Williams, I was just about to comment on the infinite zoom capabilities on CSI. That would indeed be quite a boon for astronomers! I’m also sure they would appreciate having access to those results-in-seconds relational databases, complete with overdone GUI.

The only problem is that everything in television has to make a noise. That could get pretty annoying when doing sky surveys.

Is there a preprint for this? I assume it’s Mouhcine/Ibata/Rejkuba (my main competitors! So I guess I’m supposed to say bad things about them, but they do great work, as you can see!) but I can’t actually find anything on the Subaru press release. But I might have missed it…

Going down to Magellan in a week to look at the halos of some other galaxies, in fact…

#5: The Milky Way’s stellar halo has about 1% of the total (non-dark-matter) mass. Since I can’t find the paper about M81, I’m not sure what value they’re getting, but if it were more than 5% that would be very unexpected and cool!